This invention relates to a brushless d.c. motor comprising a permanent-magnetic rotor, a stator provided with stator coils which are arranged between power supply lines, and a commutation circuit for the cyclic connection and disconnection of the power supply lines to/from a d.c. power source by means of switching devices which are controlled by the rotor magnet field. Each switching device comprises a series arrangement of a first electronic switch and a second electronic switch, which series arrangement is connectable between the positive terminal and the negative terminal of the d.c. power source. The apparatus includes a magneto-sensitive element for detecting the rotor magnet field and a control circuit for opening the first and second switches depending on the magnetic field detected by the magneto-sensitive element. Each of the power supply lines is connected to one of the junction points between the first and second switches.
Such a d.c. motor and switching device are known from U.S. Pat. No. 4,628,231.
In the known d.c. motor the opening and closing instants of the switches are derived separately for each switching device from the strength of the rotor magnet field, so that the switches of each switching device are controlled fully independently of the other switching devices. In the known motor the switching devices therefore require a minimal number of terminals, which is attractive in view of cost reduction if each switching device is wholly incorporated in an integrated circuit. However, a drawback of the known motor is that as a result of the independent control of the switches, the opening instants and the closing instants of the switches in different switching devices are not exactly contiguous. This leads to a substantial fluctuation in motor current consumption and hence to substantial fluctuations in the motor torque, which is proportional to the motor current.